Climate change is expected to cause range shifts in both terrestrial and aquatic ecosystems. However, observed range shifts have been faster in marine systems than terrestrial ones because marine populations already more fully occupy the extent of their thermal range. Rising temperatures create an urgent need for aquatic species to expand into deeper waters at higher latitudes.
Range shifts are particularly important to understand for diadromous fish, who migrate between marine and freshwater systems (ex. Atlantic salmon, brown trout, European eels). Since they need both types of habitat to stay within tolerable temperatures, there is more opportunity for changes in temperature and hydrology to impact them. When anthropogenic barriers such as dams block access to migratory routes and limit connectivity to future high quality habitat, species conservation plans can’t proceed without understanding how these forces might impact them.
A new study looks at how climate change might affect the Australian Grayling, a federally protected fish endemic to coastal rivers of south-eastern Australia that spends part of its life at sea and then returns as a juvenile to upstream freshwater habitats. The study modeled changes in habitat suitability using niche models and asked whether the inclusion of climate change affected where dam removal might be most effective. Barriers such as dams can potentially have a large impact on the Grayling’s ability to adapt to climate change by limiting connectivity to more suitable habitat upstream.
The main message is that climate change is likely to decouple connections between habitats that are critical to fish survival. Conservation planning for aquatic migratory species needs to consider both the impacts of climate change along with the potential for anthropogenic barriers such as dams to alter intersystem connectivity and prevent adaptive responses. In the case of the Australian Grayling, trailing edge populations are likely to see dramatic declines. Climate change will be the bigger threat in New South Wales and Victoria, but barriers will continue to be the biggest constraint in Tasmania. Significant warming of coastal waters and shifts in currents may contribute to changes in migratory patterns of young fish as well.
A key management strategy to support diadromous fish such as Australian Grayling will be to restore connectivity through dam removal, although this practice only makes sense when dams that act as barriers to future suitable habitat under climate change are also removed. With climate change also impacting large-scale factors such as hydrology and human water use, ensuring that threatened species can persist in highly connected freshwater systems should be a top priority.
Resources
Lin, H.-Y., A. Bush, S. Linke, H. P. Possingham, and C. J. Brown. Climate change decouples marine and freshwater habitats of a threatened migratory fish. Diversity and Distributions 23(7): 751-760.
